A thrust bearing having a bearing face with tapered lands is conventionally used for supporting thrust loads applied to the bearing face by means of a rotating shaft engaging the bearing face. The conventional thrust bearing of this type is made of a copper-type metal material containing lead or tin or an aluminum-type metal material containing tin, and is conventionally produced from a strip of the above-mentioned metal material by the employment of the press working method. As illustrated in FIGS. 1A and 1B, the conventional thrust bearing of this type has at its center a hole 1 through which will penetrate a rotating shaft with a shoulder, a bearing face 2 which is formed around the hole 1 to carry a thrust load, and a back face 3 which is formed on the opposite side of the bearing face 2. The conventional thrust bearing also has a plurality of tapered lands 4 formed in series in the bearing face 2 along the circumferential direction. The tapered lands 4 are shaped so that a wedge-shaped oil film is formed between the bearing face 2 and the surface of the shoulder of the rotating shaft. The tapered lands 4 are formed by the stamping method employing a stamping mold during the punching step by a press machine, and subsequently subjected to the well known tufftriding treatment for hardening the entire surfaces of the tapered lands 4. The tapered lands 4 are then subjected to buffing so that coarse surface of the tapered lands 4 are polished to a fine mirror surface. The tops of the tapered lands 4 are slightly rounded by the above-mentioned buffing step, although the general configuration of sharp edges of the tapered lands 4 still remain. According to such a conventional thrust bearing with tapered lands, the height of each tapered land 4 is usually from 5 to 10 microns. It is therefore difficult to form all of the tapered lands 4 maintaining the same height. Accordingly, the thrust load carried by the bearing face 2 of the thrust bearing is inevitably concentrated onto the tapered land 4 having the greatest height. Consequently, the oil film is broken on the highest tapered land 4 on which is concentrated the thrust load. Thus, so-called metal contact occurs between the highest tapered land 4 and the rotating shaft while resulting in the occurrence of abnormal wear and seizure due to high friction heat. Moreover, such an unstable factor is presented that the frictional torque changes depending upon a change in the thrust load.
In order to eliminate the above-mentioned defects, it has been proposed to flatten the tops of the tapered lands 4 by the method of polishing so that each tapered land 4 has an equal height. That is to say, one improvement over the conventional thrust bearing with tapered lands 4 has been proposed in which flat lands 5 are formed in the portions of the tapered lands 4 as shown in FIG. 2. According to the structure of tapered lands 4 having flat lands 5 as shown in FIG. 2, however, the circumferential length of the angled surface 6 for forming the wedge-shaped oil film is shortened. As a result, the surface area for carrying the thrust load is reduced. Consequently, the oil film pressure is increased for a given thrust load which will act on the oil film by means of the rotating shaft, so that the thickness of the oil film is inevitably reduced regardless of an increase in the allowable load. Accordingly, there easily takes place metal contact between the tapered lands 4 and the surface of the rotating shaft. Particularly, the thickness of the oil film is reduced at a boundary 7 between the angled surface 6 and the surface of the flat land 5, presenting the probability that the friction heat is generated at the boundary 7, which gives rise in the occurrence of seizure.